Method for changing nitrogen utilization efficiency in plants

ABSTRACT

The present invention provides a method for changing nitrogen utilization efficiency in a plant comprises regulating the expression of  Arabidopsis  NRT 1.7 or an orthologue thereof so that the nitrate remobilization from older leaves to young leaves in the plant is regulated, thereby the nitrogen utilization efficiency is changed. The present invention also provides a transgenic plant obtainable by transforming a plant with an expression construct with a high or low level of expression of NRT 1.7. On the other hand, the present invention yet provides a chimera nitrate transporter, a DNA molecule coding for this chimera transporter and an expression vector thereof.

RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application61/223,744, filed on Jul. 8, 2009, which is incorporated by reference inits entirety herein.

SUBMISSION OF SEQUENCE LISTING

The Sequence Listing associated with this application is filed inelectronic format via EFS-Web and hereby incorporated by reference intothe specification in its entirety. The name of the text file containingthe Sequence Listing is Sequence_List_(—)16024_(—)00012_US_ST25.txt. Thesize of the text file is 39 KB, and the text file was created on Jul. 8,2010.

FIELD OF THE INVENTION

The present invention is related to a method for changing nitrogenutilization efficiency in a plant by regulating expression of a nitratetransporter.

BACKGROUND OF THE INVENTION

Nitrogen fertilizer is one of the most expensive nutrients to supply.50-70% of the applied nitrogen is lost from the plant-soil system andcauses water pollution (Peoples, 1995, in: P. E. Bacon, Editor, NitrogenFertilizer in the Environment, Marcel Dekker, 565-606). Improvingnitrogen utilization efficiency (“NUE”) is important to reduce the costof crop production as well as environmental damage. Nitrogenremobilization is one of the key steps to improve NUE (Mickelson et al.,2003, J Exp Bot 54, 801-812; Masclaux-Daubresse et al., 2008, Plant Biol(Stuttg) 10 Suppl 1, 23-36).

When plants encounter nutrient deficiency, nitrogen can be recycled fromolder to younger leaves to sustain the growth of developing tissues.Nitrate remobilization occurs not only from leaf to leaf during thevegetative stage, but also from leaf to seeds during the reproductivestage. High nutrient demand during reproductive stage cannot besatisfied by Nitrogen uptake, and nitrogen recycled from senescenttissue plays an important role in sustaining grain production. Althoughseveral studies showed that nitrate remobilization was important toincrease grain yield and withstand nitrogen deprivation, little wasknown about nitrate remobilization. Thus, it is important to find outhow the stored nitrate is retrieved to withstand nitrogen deficiency andto sustain high nitrogen demand in the reproductive stage, thereby toregulate the growth of nitrogen use efficiency in a plant.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the invention relates to a discovery of Arabidopsis nitratetransporter NRT1.7 that is expressed in phloem, and is responsible forsource-to-sink remobilization of nitrate. It is unexpectedly found inthe present invention that the expression and activity of NRT1.7involves nitrate remobilization from older leaves to young leaves in theplant so as to regulate plant growth.

In one aspect, the present invention provides a method for changingnitrogen utilization efficiency in a plant comprising regulating theexpression of Arabidopsis NRT1.7 or an orthologue thereof, so that thenitrogen remobilization from older leaves to young leaves in the plantis regulated, and thereby the nitrogen utilization efficiency ischanged. According to an embodiment of the invention, a transgenic plantis prepared by transforming a plant with a construct for a high or lowlevel of expression of NRT1.7. In one example of the invention, atransgenic plant having an enhanced plant growth is prepared bytransforming a plant with an expression construct comprising a DNAsequence encoding Arabidopsis NRT1.7 or an orthologue thereof in a highlevel of expression, thereby the transgenic plant has an improvednitrate remobilization from older leaves to young leaves in the plantand nitrogen utilization efficiency. In another example of theinvention, the transgenic plant having a retarded plant growth isprepared by transforming a plant with a construct for inhibiting theexpression of NRT1.7 gene or orthologue thereof, whereby the transgenicplant has decreased nitrogen utilization efficiency.

In another aspect, the present invention provides a new chimera nitratetransporter of a NRT1.1 and NRT1.2, providing a high nitrate transportefficiency, wherein the chimera nitrate transporter has the amino acidsequence of SEQ ID NO: 11. In one embodiment of the invention, atransgenic plant having enhanced nitrogen utilization efficiency bytransforming a plant with the chimera nitrate transporter, whereby thetransgenic plant has an enhanced growth.

According to the present invention, the nitrogen utilization efficiencyin a plant is enhanced by overexpression of NRT1.7 or an enhancedexpression of the NRT 1.7, whereby the nitrate remobilization from olderleaves to young leaves in the plant is enhanced, and then the plantgrowth is improved.

The invention also provides an isolated DNA molecule encoding a chimeranitrate transporter having a amino acid sequence of SEQ ID NO:11, whichwas evidenced in Example 9 to provide high nitrate uptake so that it isbelieved that the nitrogen utilization efficiency can be enhanced. Inone example of the invention, the nucleotide sequence encoding NRT 1.7has a nucleotide sequence of SEQ ID NO: 10.

In a further yet aspect, the present invention provides a transgenicplant, which is transformed with an expression construct causingoverexpression of NRT1.7 or enhancement of NRT1.7 function within thetransgenic plant, whereby the transgenic plant enhances the nitrateremobilization from older leaves to young leaves in the plant, and thenitrogen utilization efficiency is improved. On the other hand, thepresent invention also provide a transgenic plant, which is transformedwith an construct that has a defect in the gene of NRT1.7, wherein thedefect results in inhibiting the expression of NRT1.7 or NRT1.7 mRNA todecrease quantity or availability of functional NRT1.7, whereby thenitrogen remobilization from older leaves to younger leaves in thetransgenic plant is defective.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. It should be understood, however, that theinvention is not limited to the precise arrangements andinstrumentalities shown.

In the drawings:

FIG. 1A is a diagram showing the voltage-clamped NRT1.7 cRNA-injectedoocytes demonstrating their response to 10 mM nitrate at pH 5.5 withinward current;

FIG. 1B is a diagram showing the low-affinity nitrate uptake activity ofNRT1.7-, CHL1-(NRT1.1) or water-injected oocytes, wherein the values aremeans±SD (n=5);

FIG. 1C is a diagram showing the high-affinity nitrate uptake activityof NRT1.7-, CHL1- or water-injected oocytes, wherein values are means±SD(n=10, 9, 6 for NRT1.7-, CHL1- or water-injected oocytes, respectively);

FIG. 1D is a diagram showing kinetics of nitrate-elicited currents in asingle NRT1.7 cRNA-injected oocyte determined by measuring inwardcurrent elicited by different concentrations of nitrate at pH 5.5 andplotting as a function of the external nitrate concentration; which isrepresentative of the results from six oocytes from four differentfrogs.

FIG. 2 is a diagram showing nrt1.7 expression level in vegetativetissues; wherein the number indicated rosette leaves order, andsenescence leaf (S L) is 35 days old, rosette leaves and root were 17days old grown on soil under continuous light at 23° and 65% relativehumidity; nrt1.7 expression level was high in older leaves and very lowin root.

FIG. 3A is SDS-PAGE showing the levels of NRT1.7 protein in olderleaves; wherein the top part of the same membrane was hybridized withBip antibodies as loading control, and the values of NRT1.7 proteinlevels normalized to Bip level, with the young leaves set at 1, wereindicated below the blot; and the results found in three biologicalrepeats were similar.

FIG. 3B is a diagram showing quantitative RT-PCR analysis of NRT1.7expression; wherein the leaves were separated into distal lamina,proximal lamina, and central part, including midrib and petiole, for RNAisolation; NRT1.7 was preferentially expressed in the distal part ofolder leaves; the relative expression level shown here was theexpression of NRT1.7 normalized to that of UBQ10; and the values aremeans±SE of four biological repeats; and the statistically significantdifferences were indicated by different letters (p<0.01).

FIG. 4 are images showing (A) histochemical localization of GUS activityin 28 days old pNRT1.7-GUS plants; (B) GUS activity in minor vein of a32 days old pNRT1.7-GUS plants; (C) cross section of minor veins of apNRT1.7-GUS plants; wherein GUS activity was located at the sieveelement and companion cell complex (Xy means xylem, Ph means phloem).

FIG. 5 is an image showing subcellular localization of NRT1.7 and GFPfusion protein in Arabidopsis protoplasts, wherein confocal laserscanning microscopy pictures (top panels) and corresponding bright-fieldimages (bottom panels) of Arabidopsis protoplasts transiently expressingNRT1.7-GFP, GFP-NRT1.7, or GFP alone (Bar=10 μm.

FIG. 6 is a diagram showing RNA expression levels of 20 days oldwild-type (Col) plants grown under 12/12 day/night photoperiod; whereinthe relative expression levels were the expression of NRT1.7 and NIA2normalized to the expression of UBQ10; and the values were means±SE offive biological repeats at each time point except time 0 with only twobiological repeats.

FIG. 7A provides a schematic map of the nrt1.7-1 and nrt1.7-2 mutants;wherein both mutants carried the T-DNA insertion in the second intron ofthe NRT1.7 gene; and the black and white boxes represented the codingand untranslated regions, respectively; wherein the number indicated theinsertion site of two mutants with start codon as 1 and stop codon as3890.

FIG. 7B provides the result of a western blot analysis of NRT1.7 proteinlevels in the wild-type and in homozygous nrt1.7 mutants.

FIG. 7C and FIG. 7D are diagrams showing the nitrate contentsaccumulated in old leaves of mutants; wherein the similar results wereobserved in three different pairs of Ws and nrt1.7-1 comparisons andthree of Col and nrt1.7-2.

FIG. 8A and FIG. 8B are diagrams showing nitrate remobilization from oldleaves to young leaves are defected in nrt1.7 mutants and ¹⁵N-nitratetracing assay in the wild type and mutants; wherein the amount of ¹⁵N ineach leaf is presented as the percentage of total ¹⁵N in rosette leaves;the values are mean±SE of three independent plants; and * representssignificant difference (p<0.01) between the wild types and mutants.

FIG. 8C is a diagram showing nitrate contents in phloem exudates;wherein the nitrate contents in phloem exudates of old leaf were lowerin nrt1.7 mutants; the values are mean±SE of three biological repeats;and * represents significant difference (p<0.005) between the wild typesand mutants.

FIG. 8D is a diagram showing sucrose contents in phloem exudates;wherein sucrose contents in the same phloem exudates of FIG. 8C weremeasured; there was no significant difference between the wild types andmutants; an the values were mean±SE of three biological repeats.

FIG. 9A is an image showing representative 35 days old plants grown withfull nutrients for 10 days and then nitrogen starved for 25 days.

FIG. 9B and FIG. 9C are diagrams showing rosette size of the wild typeand nrt1.7 mutants under nitrogen starvation; wherein the values weremean±SD; n=5 for Ws/nrt1.7-1, and n=4 for Col/nrt1.7-2′ and the plantswere grown with full nutrients for 10 days and the nitrogen starved for25 days for Ws background and 15 days for Col background; and *represents significant difference (p<0.005) between the wild types andmutants.

FIG. 9D is a diagram showing quantitative RT-PCR analysis of NRT1.7expression in nitrogen starved plants; wherein the plants were grownhydroponically for 34 days with full nutrients and then shifted tonitrogen-depleted medium for the time indicated; the relative expressionlevel was the expression of NRT1.7 normalized to that of UBQ10; thevalues are mean±SE of three biological repeats; and * representsSignificant difference (p<0.005) between the wild types and mutants.

FIG. 10 is a diagram showing a diagram showing the Low-affinity nitrateuptake activity of NRT1.1-, NC4N- or water-injected oocytes; and thevalues are means±SD (n=5 for water- and NRT1.1 cRNA-injected oocytes,and n=4 for NC4N cRNA-injected oocytes); and * represents significantdifference (p<0.001) between the cRNA-injected and water-injectedoocytes; # represents significant difference (p<0.05) between the NRT1.1cRNA-injected oocytes and NC4N cRNA-injected oocytes); and “NC4N” meansthe chimera gene encoding AtNRT1.1 and AtNRT1.2 fused protein.

FIG. 11 provides Arabidopsis NRT 1.7 amino acid sequences and thevarious orthologue and paralogues sequences with their percent homology.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, it is unexpectedly found that the Arabidopsisthaliana nitrate transporter NRT1.7 provides new insights into nitrateremobilization. Accordingly, the present invention provides a method forchanging nitrogen utilization efficiency in a plant comprisingregulating the expression of Arabidopsis NRT1.7 or an orthologuethereof, so that the nitrogen remobilization from older leaves to youngleaves in the plant is regulated, and thereby the nitrogen utilizationefficiency is changed.

The term “Arabidopsis NRT1.7” or “NRT1.7” as used herein refers toArabidopsis nitrate transporter NRT1.7, having the amino acid sequenceof SEQ ID NO:2, or a protein encoded by a nucleic acid sequence of SEQID NO:1.

The term “orthologue” used herein refers to one of two or morehomologous gene sequences found in different species. In the invention,the orthologue of Arabidopsis thaliana nitrate transporter NRT1.7includes but not limited to any transporter having an amino acidsequence that is at least 40% homologous to the consensus amino acidsequence of NRT1.7 (such as the transporter having the amino acidsequence of SEQ ID NO: 2), preferably at least 60%, most preferably 80%homologous to the consensus amino acid sequence of NRT1.7, such as thoseshown in FIG. 11.

According to an embodiment of the invention, a transgenic plant isprepared by transforming a plant with an expression construct for a highor low level of expression of NRT1.7. In one example of the invention, atransgenic plant having an enhanced plant growth is prepared bytransforming a plant with an expression construct comprising a DNAsequence encoding Arabidopsis NRT1.7 or an orthologue thereof in a highlevel of expression, thereby the transgenic plant has an improvednitrate remobilization and nitrogen utilization efficiency. In anotherexample of the invention, the transgenic plant having a retarded plantgrowth is prepared by transforming a plant with an expression constructcomprising null mutation of the NRT1.7 gene, whereby the transgenicplant has a decreased nitrogen utilization efficiency.

Based on the several quantitative trait locus analyses as obtained, thegrain yield and nitrogen utilization efficiency were well correlatedwith nitrate storage capacity and efficient remobilization. Westernblotting, quantitative RT-PCR, and □-glucuronidase reporter analysis asobtained in the present invention indicated that NRT1.7 was expressed inthe phloem of the leaf. In nrt1.7 mutants, more nitrate was present inthe older leaves, less ¹⁵NO₃— spotted on old leaves was remobilized intoN-demanding tissues, and less nitrate was detected in the phloemexudates of old leaves. Meanwhile, nrt1.7 mutants also showed growthretardation when external nitrogen was depleted. It is concluded thatnitrate remobilization is important to sustain vigorous growth duringnitrogen deficiency, and the nitrogen utilization efficiency can bechanged by regulating the expression of NRT1.7 in a plant.

According to the present invention, a method of enhancing nitrogenutilization efficiency in a plant comprises overexpressing NRT1.7 orenhancing NRT1.7 function in a plant, wherein the nitrogenremobilization from older leaves to young leaves in the plant isenhanced, thereby the nitrogen utilization efficiency is improved.

In the invention, NRT1.7 gene expression is regulated to produce moreNRT1.7 protein than ordinary conditions in the corresponding wild typeplants. Methods for overexpression of a protein in vivo are well knownin the art. Thus the invention also encompasses all possible methodologyfor overexpression NRT1.7 gene or modulating activity of NRT1.7 proteinin a plant, including regulation of transcription and post-translationregulation. For example, the modulating of gene expression may be used,i.e. by modulating the expression of the gene itself by a suitablepromoter and/or a transcription enhancer or a translation enhancer.Alternatively, the modulation of expression as mentioned above iseffected in an indirect way, for example as a result of increased levelsand/or activity of factors that control the expression of NRT1.7 gene.In one example of the invention, the enhancer may be used to enhancetranscription levels of genes in a gene cluster.

According to the invention, any enhancer for enhancing the expression ofNRT1.7 may be used to prepare an expression construct for transforming aplant to produce a transgenic plant. For example, CaMV 35S enhancers(Weigel et al., Plant Physiol. 122(4):1003-1013. 2000, April) may beused for enhancing the expression of NRT1.7. In one example of thepresent invention, 35S enhancer (SEQ ID NO:4) can be modified to linkwith NRT1.7 promoter (SEQ ID NO:3) or inserted into downstream of NRT1.7coding region alone. In one embodiment of the invention, 35S enhancer isoperatedly linked with NRT1.7 promoter to produce an artificial nucleicacid sequence of SEQ ID NO: 5. One can prepare an expression constructcomprising the chimera DNA sequence of SEQ ID NO: 5, inserted to anexpression construct to overexpress NRT1.7.

According to the invention, if a transgenic plant has the overexpressionof NRT1.7 or enhancing the activity of NRT1.7, the nitrogenremobilization from older leaves to younger leaves in the transgenicplant will be enhanced; and accordingly it is believed that thetransgenic plant has faster growth and higher yield. Therefore, thepresent invention provides a transgenic plant transformed with anexpression construct comprising a nucleic acid sequence causing a highlevel of expression, such as overexpression, of NRT1.7 or enhancement ofNRT1.7 function within the transgenic plant, wherein expression of theDNA molecule in the transgenic plant enhances the nitrogenremobilization from older leaves to young leaves in the plant, therebythe nitrogen utilization efficiency is improved.

The phrase/clause “faster growth” or “the growth is enhanced” usedherein refers to the increase either in weight or size, for examplefresh weight, or in biomass per time unit is greater that that of theplant of same species.

The term “yield” used herein refers to the amount of harvested materialper area of production. The term “higher yield” means an increase inbiomass in one or more parts of a plant relative to that ofcorresponding wild type plants. The harvested parts of the plant can besuch as seed (e.g. rice, sorghum or corn), root (e.g. sugar beet), fruit(e.g. apple), flowers, or any other part of the plant, which is ofeconomic value.

Transformation of a plant species is now fairly routine technique.Advantageously, any of several transformation methods may be used tointroduce the gene of interest into a suitable ancestor cell.Transformation methods include, but not limited to, the use ofliposomes, electroporation, chemicals that increase free DNA uptake,injection of the DNA directly into the plant, particle gun bombardment,viruses or pollen and microinjection. In one embodiment of the presentinvention, plant transformation was performed as described in Clough etal, 1998, Plant J 16, 735-743.

It is found in the present invention that NRT1.7 involves in nitrateremobilization from the old leaves to young leaves in a plant. In oneexample of the present invention, a mutant of nrt1.7 defective in thisprocess was prepared to retard the plant growth when the plantsencountered long-term severe nitrogen deficiency during vegetativegrowth. It was indicated that internal nitrate remobilization betweenleaves was important for plants to cope with nitrogen deficiency and theimportance of enhanced nitrogen use efficiency for maximum growth. Thepresent invention further provides a method for retarding growth in aplant comprising decreasing quantity or activity of NRT1.7, orinhibiting the expression of a gene encoding NRT1.7 within the plant,thereby causes the defect in remobilizing nitrogen from older leaves toyounger leaves so as to retard growth in the plant.

Techniques for decreasing quantity or activity of a protein, orinhibiting the expression of a gene in vivo are also well known andenvisaged in the art, whether by a direct or indirect approach. Examplesof decreasing expression includes, but not limited, by anti-sensetechniques, co-suppression techniques, RNAi techniques, smallinterference RNAs (siRNAs), micor RNA (miRNA), the use of ribozymes,etc. According to one embodiment of the present invention, the growth ofa plant was modified by introducing into a plant an additional copy (infull or in part) of a NRT1.7 gene fragment already present in a hostplant. The additional gene silences the endogenous gene, giving rise toa phenomenon known as co-suppression. In another embodiment of thepresent invention, gene silencing may also be achieved by insertionmutagenesis, e.g., T-DNA insertion or transposon insertion, or by genesilencing strategies as described in published prior arts.

The present invention also provides a transgenic plant obtainable by themethods for decreasing quantity or activity of a protein, or inhibitingthe expression of a gene in vivo above. In one example of the invention,the transgenic plant had defects in the gene of NRT1.7, wherein thedefects in the gene resulted in inhibiting the expression of NRT1.7 mRNAor proteins to decrease quantity or availability of functional NRT1.7,thereby the nitrogen remobilization from older leaves to younger leavesin the transgenic plant is defective. According to the invention, suchtransgenic plant performs growth retardation during nitrogen starvation.

The present invention provides a new chimera nitrate transporter havingthe amino acid sequence of SEQ ID NO: 11, and the DNA molecule havingthe sequence coding for this chimera protein. In one example of theinvention, an isolated DNA molecule encoding a chimera nitratetransporter named as NC4N is provided, which comprises the nucleotidesequence of SEQ ID NO:10, coding for the chimera nitrate transporterhaving the amino acid sequence of SEQ ID NO: 11. According to theinvention, the chimera protein is prepared from NRT1.1 and NRT1.2 withNRT1.2-NRT1.1-NRT1.2 shuffling form, in which at the residues of 76-195positions of NRT1.2 amino acid sequences (SEQ ID NO: 9) was replaced byat the residues of 78-200 positions of NRT1.1 amino acid sequences (SEQID NO: 7).

Arabidopsis NRT1.1 and NRT1.2 participate in nitrate uptake using aproton gradient as a driving force to transport nitrate from the soilinto plant cells. Unexpectedly, the inventors found that the chimeraprotein performed better activity on nitrate uptake than wild typeNRT1.1 and NRT1.2. In one embodiment of the invention, functionalAnalysis of the chimera protein was determined by Xenopus laevis oocytestest as described in the Example 9. As evidenced in FIG. 10, both NRT1.1cRNA-injected oocytes and NC4N cRNA-injected oocytes were found to takeup more nitrate than water-injected oocytes. Moreover, NC4NcRNA-injected oocytes were found to take up more nitrates than NRT1.1cRNA-injected oocytes. The results indicate that the chimera fusedprotein of the present invention has better transport activity than anyknown NRT transports.

Furthermore, the present invention provides a method enhancing nitrogenutilization efficiency in a plant comprising transforming a plant withthe DNA molecule encoding the claimed chimera transporter having theamino acid sequence of SEQ ID NO: 11 to produce a transgenic plant,wherein the nitrogen utilization efficiency is enhanced in thetransgenic plant, and subsequently, the transgenic plant has fastergrowth and higher yield. Preferably, the DNA molecule encoding theclaimed chimera transporter is driven by a NRT1.7 promoter in thetransgenic plant.

The present invention is further illustrated by the following examples,which are provided for the purpose of demonstration rather thanlimitation.

EXAMPLES Methods and Materials

Functional Analysis of NRT1.7 in Xenopus laevis Oocytes

A full length cDNA fragment of NRT1.7 (SEQ ID NO:1) was cloned into thepGEMHE vector (Liman et al., 1992, Neuron 9, 861-871) to generatepGEMHE-NRT1.7. The pGEMHE-NRT1.7 was linearized using NheI, and cappedmRNA was transcribed in vitro using mMESSAGE mMACHINE kits (Ambion).Oocytes were injected with 100 ng of NRT1.7 cRNA as described previously(Tsay et al., 1993, Cell 72, 705-713). Electrophysiological analyses ofinjected oocytes were performed as described previously (Huang et al.,1999, Plant Cell 11, 1381-1392). Nitrate uptake assays using ¹⁵N-nitratewere performed as described previously using a continuous-flow isotoperatio mass spectrometer coupled with a carbon nitrogen elementalanalyzer (ANCA-GSL MS; PDZ Europa; (Lin et al., 2008, Plant Cell 20,2514-2528)), and oocytes injected with CRL1 cRNA (Liu et al., 1999,Plant Cell 11, 865-874) were used as a positive control.

Plant Growth Condition and nrt1.7 Mutants

Unless otherwise indicated, most Arabidopsis thaliana plants used inthis study were grown in soil containing compost:humus at 3:1, at 22°C., with 16-hr photoperiod, and 60% relative humidity, and irrigatedwith HYPONeX #2 fertilizer at final concentrations of 6 mM nitrate, 5.3mM potassium, and 3.5 mM phosphate. For nitrogen starvation experiments,plants were grown in soil containing perlite:vermiculite at 1:2 andcovered with a thin layer of fine vermiculite, irrigated with HYPONeX #2fertilizer for 10 or 25 days as indicated in the figure legends, andthen watered with a nitrogen-depleted solution containing 5 mMK₂HPO₄/KH₂PO₄, pH 5.5, and the basal nutrients (1 mM MgSO₄, 0.1 mMFeSO₄-EDTA, 0.5 mM CaCl₂, 50 μM H₃BO₃, 12 μM MnSO₄, 1 μM ZnCl₂, 1 μMCuSO₄.5H₂O, and 0.2 μM Na₂MoO₄.2H₂O). For phloem exudates collection andmeasurement of NRT1.7 expression in response to starvation, plants weregrown hydroponically in a solution containing 1 mM K₂HPO₄/KH₂PO₄ at pH5.5, the basal nutrients described above and with or without 1 mMNH₄NO₃. All experiments compared wild-type and mutant plants grown inthe same pot.

The nrt1.7-1 was obtained from the ALPHA population (WS ecotype) ofT-DNA-tagged plants generated by the Arabidopsis Knockout Facility atthe University of Wisconsin Biotech Center (Krysan et al., 1999, PlantCell 11, 2283-2290). The primers used for PCR screening were JL202 (Linet al., 2008, Plant Cell 20, 2514-2528) and the NRT1.7 forward primer(SEQ ID NO:12: 5′-CCACACCCACCATATATTATCTACTCACT-3′). The second mutantnrt1.7-2 (SALK_(—)053264) was provided by the Salk Institute GenomicAnalysis Laboratory (Alonso et al., 2003, Science 301, 653-657).

Antibody and Western Blot

The anti-NRT1.7 rabbit polyclonal antibody was generated using a peptidecorresponding to the first N-terminal 50 amino acids. The cDNA fragmentencoding the N-terminal 1-50 a.a. was amplified by PCR using primerspair of (SEQ ID NO:13: forward 5′-gaattctaATGGTTTTGGAGGATAG-3′ and SEQID NO:14: reverse 5′-aaGCTTTTTCTCTACCTTCTCAG-3′), which introduced EcoRIand HindIII restriction sites respectively, and subcloned into pGEX-KGin frame with the GST to generate pGEX-KG-NRT1.7-N50. GST-fusion proteinwas isolated from E. coli (BL21) transformant and purified by GST-beads.Purified GST-fusion protein was emulsified with Freund's adjuvant andinjected into New Zealand rabbits according to the protocol of Spindleret al. (Spindler et al., 1984, J Virol 49, 132-141).

For protein gel blot analysis, tissues were homogenized in ice coldextraction buffer consisting of 15 mM Tris-HCl, pH 7.8, 250 mM sucrose,1 mM EDTA, 2 mM DTT, 1 mM phenylmethylsulfonyl fluoride, 0.6%polyvinylpyrrolidone, and protease inhibitor cocktail (Roche). Thehomogenate was then centrifuged at 10,000×g for 10 min and thesupernatant was collected into a chilled tube. The supernatant wascentrifuged at 100,000×g for 1 h, and then the pellet, the microsomalfraction, was dissolved in 4% SDS. 10 micrograms of protein wereanalyzed by SDS-PAGE. Detections were performed using the ECL proteingel blotting system (Amersham, GE Healthcare, UK). Anti-NRT1.7, Anti-BiPand horseradish peroxidase-labeled anti-rabbit IgG antibody were used atdilutions of 1:2000, 1:2000 and 1:10000, respectively.

RT-PCR and Quantitative RT-PCR

The ImProm-II reverse transcriptase (Promega), oligo(dT) primers, andthe RNA isolated from different developmental stages of leaves andflowers were used to synthesize the first-strand cDNAs. Primers acrossthe intron of Histone were used to exclude genomic contamination.Primers specific for the NRT1.7, NIA2 and UBQ10 gene were designed byABI software. Quantitative PCR was performed in AB7500 using Power SYBRGreen (ABI System). The primers used were as follows:

TABLE 1 Primer Sequence SEQ ID Gene Sequence NO: Histone (Forward)5′-AACCACTGGAGGAGTCAAGA-3′ 15 Histone (Reverse)5′-CAATTAAGCACGTTCTCCTCT-3′ 16 NRT1.7 (Forward)5′-CAACAGTCAGTTTCCAGAGCACAT-3′ 17 NRT1.7 (Reverse)5′-CGACAGTCACAAGGAAACTACTAAGGTA-3′ 18 NIA2 (Forward)5′-AGGATCCAGAGGATGAGACTGAAA-3′ 19 NIA2 (Reverse)5′-CCTTAGCTGATTCCACTACGTACCA-3′ 20 UBQ10 (Forward)5′-AGAAGTTCAATGTTTCGTTTCATGTAA-3′ 21 UBQ10 (Reverse)5′-GAACGGAAACATAGTAGAACACTTATTCA-3 22

Promoter-GUS Analysis

A 1.35-kb genomic fragment of NRT1.7 promoter (−1346 to +3 bp) wasgenerated by PCR using the primers forward5′-gtcgaCAAATATTTTCCTATAACATA-3′ (SEQ ID NO: 23:) and reverse5′-ggatcctCATCTCTAAGATATTACT-3′ (SEQ ID NO: 24), cut with XbaI andBamHI, and then inserted in-frame in front of uidA (GUS) of pBI101.Plant transformation was performed as described (Clough et al, 1998,Plant J 16, 735-743). Homozygous transgenic plants (T3) 28-32 days oldcultivated in soil with full nutrient were used for GUS histochemicalassay, with GUS staining as described previously (Lagarde et al., 1996,The Plant Journal 9, 195-203). Cross-sections of 2 μm thickness wereprepared using a microtome (Ultracut E, Reichert-Jung) from tissuesembedded in LR white.

Whole-Mount Immunolocalization

To enhance the specificity, anti-NRT1.7 antiserum was affinity purifiedfirst by the antigen used to raise antiserum (a GST fusion of the first50 amino acids of NRT1.7) and then by HA-tagged full-length NRT1.7protein. Older (40 d old) Col and nrt1.7-2 leaves were used for wholemount immunohybridization. For antigen retrieval before hybridization,tissues were incubated in 1 mM EDTA at 95° for 5 min and then blockedfor 2 hours in blocking buffer (50 mM Tris-HCL, pH 7.5, 150 mM NaCl, and1% gelatin). After 36 hours incubation with affinity-purifiedanti-NRT1.7 antiserum in 1:10 dilution at 4°, tissues were washed threetimes with blocking buffer and then hybridized with Alexa Fluor 488 goatanti-rabbit IgG (Molecular Probes) in 1:500 dilution. Green fluorescencewas detected by a Zeiss LSM META-510 microscope with excitation at 488nm. Fluorescence emission signals were detected using a band-pass filterof 505 to 530 nm Sieve plates were stained with 0.2% aniline blue (WaterBlue; Fluka) in 50 mM Na—PO₄ buffer for 30 min. Aniline bluefluorescence was detected with an excitation light of 405 nm andband-pass filter of 420 to 480 nm.

GFP Fusion and Subcellular Localization

To construct the plasmid encoding NRT1.7-GFP fusion protein, NRT1.7 cDNAwas amplified by PCR using the primers NRT1.7NF (SEQ ID NO:25:5′-tctagATGGTTTTGGAGGATAGA-3′) and NRT1.7NR (SEQ ID NO:26:5′-ggatccCATTTCATCGATTTCTT-3′); the former primer introduces a XbaIrestriction site and the latter removes the stop codon and introduces aBamHI restriction site. The amplified DNA fragment was then cloned inframe in front of the GFP coding region in the vector 326-GFP, leadingto the final pNRT1.7-GFP construct under the control of the 35Spromoter. The fusion linker between NRT1.7 and GFP contained seven aminoacids (YIQGDIT). To construct the plasmid encoding pGFP-NRT1.7 fusionprotein, the NRT1.7 cDNA was amplified by PCR with primers NRT1.7CF (SEQID NO:27: 5′-ctcgagATGGTTTTGGAGGATAGA-3′ and NRT1.7CR (SEQ ID NO:28:5′-ctcgagTCATTTCATCGATTTCTT-3′) which introduced XhoI restriction sites,then cloned in frame into vector 326-GFP-nt (no termination codon)behind the GFP. The fusion linker between GFP and NRT1.7 containedthirteen amino acids (PRAIKLIDTVDLE). The vector 326-GFP was used as afree GFP control.

Transient transformation of Arabidopsis protoplasts with polyethyleneglycol was performed as described (Yoo et al., 2007, Nat Protoc 2,1565-1572). After transformation, protoplasts were incubated overnightat room temperature under illumination (25 μE), and then observed by aZeiss LSM510 microscope with excitation at 488 nm Fluorescence emissionsignals were detected using a band-pass filter of 500 to 530 nm for GFPand a long-pass filter of 650 nm for the far-red autoflourescence of thechloroplast.

Measurement of the Nitrate Content in Arabidopsis Leaves

The rosette leaves were collected and immediately frozen in liquidnitrogen. To extract nitrate, samples were boiled in water (100 μl/mgFW) and then freeze-thawed once. After filtering through 0.2 μm PVDFmembrane (Pall Corporation), nitrate content of the samples wasdetermined by HPLC using a PARTISIL 10 SAX (strong anion exchanger)column (Whatman) and 50 mM phosphate buffer, pH 3.0, as the mobilephase.

¹⁵NNitrate Tracing Assay

Three days after bolting, 10 μl of 50 mM K¹⁵NO₃ with a 98% atom excessof ¹⁵N was spotted on distal parts of the oldest leaf. About 20 hr afterspotting, individual leaves and flowers were collected and dried at 80°C. for 24 hr., at which point ¹⁵N contents were analyzed as describedabove.

Collection and Analysis of Phloem Exudates

Three days after bolting, phloem exudates were collected from excisedleaves using procedures modified from the protocol described by Deekenet al. (Deeken et al., 2008, Plant J 55, 746-759). The third and fourthleaves were cut and the tip of the petiole was re-cut in EDTA buffer (5mM Na₂EDTA, pH 7.5, osmotically adjusted to 270 mOsmol with sorbitol)with fresh razor blades without wounding. The leaves were washed with alarge volume of sterile EDTA buffer to remove contaminants and thenplaced in 200 μl new EDTA buffer. During phloem sap exudation, theleaves were illuminated (25 μE) and incubated in CO₂— and H₂O-saturatedair. After 1 h of bleeding, the buffer solution containing phloemexudates were analyzed for nitrate and sugar content. Nitrate contentswere measured by HPLC as described above. Sucrose and glucose contentwere measured by the DNS method as described elsewhere (Bernfeld, 1995,Methods Enzymol. 1, 149-158).

Construction of Chimera NRT Protein by Fusing NRT1.1 and NRT1.1

pGEMHE-05N was made by replacing 1-701 nucleotides of AtNRT1.2 (SEQ IDNO: 8) with AtNRT1.1 (SEQ ID NO:6). The shuffling region was generatedby PCR using pGEMHE-AtNRT1.1 as template and T7 (AATACGACTCACTATAG) (SEQID NO: 29) and primer1 (GGCTACTAGTGCGCCAACGTTGATACAA) (SEQ ID NO: 30) asprimer set and digestion by BamHI and SpeI.

pGEMHE-NC4N was made by replacing 1-231 nucleotides of pGEMHE-CSN withAtNRT1.2. The N-terminal region of pGEMHE-NC4N was generated by 1^(st)PCR, using pGEMHE-AtNRT1.2 as the template and primer2(CCCGGATCCGAATGGAAGTGGAAGAAG) (SEQ ID NO: 31) and primer3(AGAAGTTCCGAGGAAATTGGTGACGTCATTTGCCGA) (SEQ ID NO: 32) as the primerset. The C-terminal region of pGEMHE-NC4N was made by 2^(nd) PCR withpGEMHE-05N as template, and primer4 (TCGGCAAATGACGTCACCAATTTCCTCGGAACTTCT) (SEQ ID NO: 33) and primer 5(CCCGAATTCTTTAGCTTCTTGAACCAG) (SEQ ID NO: 34) as the primer set. The3^(rd) PCR of pGEMHE-NC4N construction was done by using the product of1^(st) and 2^(nd) PCR products as the template and primer2 and primer 5as the primer set. The chimeric fragment (SEQ ID NO: 10) was digested byBamHI and EcoRI and then ligated into pGEMHE vector to obtainpGEMHE-NC4N.

In order to make the pGEMHE constructs with HA tag, the AtNRT1.1-HAfragment was done by PCR with pGEMHE-AtNRT1.1 as the template, primer6(CCCGGATCCAAAACAGCCTTTTACATA) (SEQ ID NO: 35) and primer 7(CCCGAATTCTCAAGCGTAATCTGGAACATCGTATGGGTACCCCCCATGACCCA TTGGAATACTCG)(SEQ ID NO: 36) as primer set; NC4N-HA was done by PCR with pGEMHE-NC4Nas the template primer2 and primer8(CCCGAATTCTTAAGCGTAATCTGGAACATCGTATGGGTACCCCCCGCTTCTTG AACCAGTTGATC)(SEQ ID NO: 37) as the primer set. The fragments with HA tag weredigested by BamHI and EcoRI and then ligated into pGEMHE vector.

The final NC4N chimera protein has 588 amino acids represented by SEQ IDNO:11 with NRT1.2-NRT1.1-NRT1.2 shuffling form, in which at the residuesof 76-195 positions of NRT1.2 amino acid sequences (SEQ ID NO: 9) wasreplaced by at the residues of 78-200 positions of NRT1.1 amino acidsequences (SEQ ID NO: 7).

Accession Numbers

Sequence data enclosed herein can be found in the GeneBank/EMBL datalibraries under the following accession numbers: At1g69870 (NRT1.7),At1g12110 (CHL1, NRT1.1), At1g69850 (NRT1.2), At3g21670 (NTP3, NRT1.3),At2g26690 (NTP2, NRT1.4), At1g32450 (NRT1.5), At1g27080 (NRT1.6),At1g08090 (NRT2.1), At1g08100 (NRT2.2), At1g12940 (NRT2.7), At3g45650(NAXT1), At3g54140 (PTR1), At2g02040 (PTR2), At5g46050 (PTR3), At5g40890(CLCa), At5g40780 (LHT1), At4g05320 (UBQ10), At1g22710 (SUC2), At4g22200(AKT2), At4g40040 (Histone), and At5g42020 (BiP).

Example 1 NRT1.7 Encodes a Low Affinity Nitrate Transporter

In Arabidopsis, there are 53 NRT1 (PTR) genes, some of which are knownto transport nitrate, while others transport dipeptides. To determinethe substrate specificity of NRT1.7, in vitro-synthesized cRNA wasinjected into Xenopus oocytes for electrophysiological analysis. After2-day incubation in ND96, oocytes were voltage clamped at −60 mV, andthen subjected to 300 ms voltage pulses from 0˜160 mV in −20 mVincrements. NRT1.7-injected oocytes responded to 10 mM nitrate at pH 5.5with inward currents. And the inward currents were elicited by nitratebut not by the dipeptides tested (FIG. 1A). The current elicited bynitrate was pH-dependent, with little or no current detected whenexposed to nitrate at pH7.4. The pH dependence of the nitrate elicitedcurrents suggested that NRT1.7 is a proton-coupled nitrate transporter.

Most of the nitrate transporters in NRT1 (PTR1) family function as alow-affinity transporter with exception of NRT1.1 (CHL1), which is adual-affinity nitrate transporter. To determine the affinity of NRT1.7,the high- and low-affinity nitrate transport activities of cRNA-injectedoocytes were assessed by incubating the oocytes with 10 mM ¹⁵N-nitratefor 2 hours or 250 μmM ¹⁵N-nitrate for 1 hour, respectively. Consistentwith the previous data, CHL1 cRNA-injected oocytes showed both high- andlow-affinity nitrate transport, while NRT1.7 cRNA-injected oocytes werefound to take up nitrate only with low affinity (FIGS. 1B and 1C). TheK_(m) of NRT1.7 for nitrate was calculated from currents elicited at −60mV by different concentrations of nitrate. The average K_(m) calculatedfrom 6 independent oocytes was 2.7±0.6 mM (FIG. 1D).

Example 2 NRT1.7 is Expressed in Phloem Tissue of Old Leaves

Microarray data from the public resource A. thaliana Expression DatabaseCSB.DB shows that little or no expression of NRT1.7 can be detected inroot, and that transcription levels in leaves increased as leaves age(FIG. 2). The differential expression of NRT1.7 in old and young leaveswas further confirmed here by Western Blot analysis (FIG. 3A). Using BiPas loading control, the NRT1.7 protein level in the oldest leaves wasabout 25 times higher than that in the youngest leaves. In addition, theleaves were separated into distal lamina, proximal lamina, and centralpart including midrib and petiole for quantitative RT-PCR analysis. TheNRT1.7 mRNA level was higher in the distal lamina of older leaves (FIG.3B).

To determine where NRT1.7 is expressed, 13 independent transgenic linesexpressing GUS driven by NRT1.7 promoter were analyzed. Consistent withWestern Blot result, GUS staining was stronger in the older leaves,while no staining was detected in the younger leaves. In between, therewere a few transition leaves with GUS staining extending from the tip tothe base of the leaves (indicated by arrows in FIG. 4A). A similarpattern was found in all of the 13 independent lines. This expressionpattern of NRT1.7 suggested that it might be involved in phloem loading,particularly in matured leaves.

Closer examination of the GUS staining indicated that NRT1.7 was mainlyexpressed in minor veins (FIG. 4B). In addition, microscopic analysis ofthe leaf sections indicated that the expression was restricted to thesieve element and companion cell complex (FIG. 4C).

Example 3 NRT1.7 is Localized to the Plasma Membrane

To investigate the subcellular localization of NRT1.7, green fluorescentprotein (GFP) fused either N-terminally or C-terminally to NRT 1.7 wastransiently expressed in Arabidopsis protoplasts under the control ofthe cauliflower mosaic virus 35S promoter. Green fluorescence was seenin cytoplasm in the GFP control, while the green fluorescence ofNRT1.7-GFP and GFP-NRT1.7 (FIG. 5) was detected as a fine ring at thecell periphery, external to the chloroplasts, indicating that NRT1.7 islocalized in the plasma membrane.

Example 4 Expression of NRT1.7 is Diurnally Regulated and TemporallyOpposite to that of NIA2

Shoot of plants grown under 12/12 day/night cycle for 20 days werecollected to determine the diurnal changes in the expression of NRT1.7and nitrate reductase gene NIA2. Q-PCR analysis indicated that theNRT1.7 transcript level increased gradually during the light period,reached a maximum in the early part of the dark period, and declinedthereafter (FIG. 6). In contrast, NIA2 transcript levels decreasedduring light period, were minimal at the late stage of the light period,and then increased gradually during the dark period. This oppositetemporal pattern of NRT1.7 and NIA2 mRNA levels suggests that NRT1.7 isneeded when NR activity is low.

Example 5 nrt1.7 Null Mutants Accumulate Higher Amount Nitrate in OlderLeaf

To determine the in vivo function of nrt1.7, two T-DNA insertion mutantswere isolated. Mutant nrt1.7-1 in the Wassilewskija (WS) ecotype wasisolated by PCR-based screening (Krysan et al., 1999, Plant Cell 11,2283-2290), and a second mutant nrt1.7-2, SALK 053264, in the Columbia(Col) ecotype was obtained from ABRC (Alonso et al., 2003, Science 301,653-657). In nrt1.7-1 and nrt1.7-2 mutants, one copy and threecontiguous copies of T-DNA, respectively, were inserted in the secondintron of NRT1.7 gene (FIG. 7A). No expression of NRT1.7 mRNA andprotein could be detected by RT-PCR (data not shown) and Western blotanalysis (FIG. 7B) showing that both are null mutants

The nitrate content in each leaf was analyzed in wild type and mutants.Compared to the wild type, higher amounts of nitrate accumulated in oldleaves of the mutants (FIGS. 7C and 7D). Preferential expression ofNRT1.7 in old leaves and accumulation of nitrate in the old leaves ofthe mutants suggest that NRT1.7 is responsible for remobilizing nitratefrom the old leaves to other tissues.

Example 6 nrt1.7 Mutants were Defective in Remobilizing ¹⁵N-Nitrate fromOld Leaves to Young Leaves and Flower

That NRT1.7 functions in nitrate remobilization was further confirmed bya ¹⁵N-nitrate spotting experiment. ¹⁵N-nitrate was spotted on distalparts of the oldest non-senescent leaf, and 20 hours after spotting, ¹⁵Ncontents of different leaves and organs were analyzed. In the wild type,¹⁵N-nitrate spotted on the old leaf moved to young leaves; in themutants, little or no ¹⁵N could be found in the young leaves (FIGS. 8Aand 8B). These data indicate that the nitrate transporter NRT1.7 isresponsible for remobilizing nitrate from older leaves to N-demandingtissues, such as young leaves.

Since NRT1.7 is expressed in the phloem of old leaves, the amount ofnitrate in the phloem sap was compared between wild type and mutants. Ina slight modification of an older protocol (Deeken et al., 2008, Plant J55, 746-759), the third and fourth leaves were cut, recut in EDTAbuffer, washed and then placed into tubes with 200 μl EDTA buffer. Afterphloem bleeding for 1 h, the buffer solution, which contained dilutedphloem sap, was used for composition analyses. The glucose content inthe phloem exudates was lower than the detection limit (50 nmole/g freshweight [FW]), suggesting that the concentration of damaged cell extractin the exudates was low. Nitrate contents in the exudates were 159.9±9.7n mole/g FW in WS, 96.8±7.4 in nrt1.7-1; 193.2±11.5 in Col, and123.9±5.0 in nrt1.7-2 (FIG. 8C) indicating that compared to wild type,the nitrate contents of phloem exudates in nrt1.7 mutants decreased35˜40%. Other types of transporters or loading mechanisms could beresponsible for the remaining nitrate detected in the phloem sap of thenrt1.7 mutants. The sucrose content in the mutants is comparable to thevalues of their corresponding wild types (FIG. 8D), suggesting thatreduced nitrate content in the mutants is not due to reduced exudationrate of phloem sap in the mutants.

Example 7 Growth Retardation in nrt1.7 Mutants During NitrogenStarvation

Under nutrient-sufficient conditions, no growth difference was seenbetween mutants and wild type. However, when plants were starved ofnitrogen at an early stage (10 days after germination), compared to wildtype, both nrt1.7 mutants showed growth retardation (FIG. 9A). Whencompared with the wild type grown in the same pots, the mutant rosetteswere about 30% smaller in diameter (FIGS. 9B and 9C). QuantitativeRT-PCR analysis revealed that NRT1.7 expression was induced by nitrogenstarvation (FIG. 9D). The growth retardation found in mutants andenhanced expression of NRT1.7 by nitrate starvation suggests thatnitrate remobilization is important to sustain vigorous growth undernitrogen-starvation conditions.

Example 8 Preparation of Transgenic Plants Overexpressing NRT1.7 Proteinto Enhance the Growth

According to the studies of Example 8, nrt1.7 gene mutation can resultin the growth retardation of the transgenic plant. From this fact, itreasonably deduces that overexpression of NRT1.7 in a plant mightenhance nitrate remobilization thereby the growth of the plant isenhanced and resistant to nitrogen starvation.

To this aim, i.e., the expression of NRT1.7 can be put under the controlof its own promoter operatedly linked with 35S enhancer (see Weigel etal., Plant Physiol. 122(4):1003-1013. 2000, April). Plant transformationwas performed as described (Clough et al, 1998, Plant J 16, 735-743).Briefly, NRT1.7 promoter was eluted from NRT1.7 promoter-GUS bydigesting with BamHI and XbaI as described above. NRT1.7 cDNA wasgenerated by PCR using pGEMHE-AtNRT1.7 as the template and the primersforward 5′-ATCAAGCTTGCTCTAGAG-3′ (SEQ ID NO: 38) and reverse5′-GGGATCCAGATGGTTTTGGA-3′ (SEQ ID NO: 39). The PCR product was cut withXbaI and BamHI. The XbaI ligated fragment containing NRT1.7::NRT1.7 wasinserted into a mini-binary vector pCB302 for transform into ArabidopsisCol wild type and nrt1.7-2. Another binry vector, pSKI015, with 4×35Senhancer (SEQ ID NO: 4) was digested with SpeI and ligated with the XbaIfragment containing NRT1.7::NRT1.7 to obtain an expression vector. Atransgenic plant overexpression of NRT1.7 shall be obtainable bytransforming this expression vector in which.

Example 9 AtNRT1.1 and AtNRT1.2 Fused Protein (NC4N) Exhibiting GreaterTransport Activity

To determine the transport activity of the fused protein (NC4N) in vivo,a full length cDNA fragment encoding AtNRT1.1 and AtNRT1.2 fused proteinwas cloned into the pGEMHE vector to generate pGEMHE-NC4N. ThepGEMHE-NC4N was linearized using NheI, and capped mRNA was transcribedin vitro using mMESSAGE mMACHINE kits (Ambion). Oocytes were injectedwith 50 ng of NC4N cRNA as described previously. Nitrate uptake assaysusing 10 mM ¹⁵N-nitrate were performed as described previously using acontinuous-flow isotope ratio mass spectrometer coupled with a carbonnitrogen elemental analyzer, and oocytes injected with NRT1.1 cRNA orwater (H₂O) were used as a positive or negative control, respectively.The result was shown in the FIG. 10. The values are mean±SD; n=5 forwater- and NRT1.1 cRNA-injected oocytes, and n=4 for NC4N cRNA-injectedoocytes. * represents significant difference (p<0.001) between thecRNA-injected and water-injected oocytes. # represents significantdifference (p<0.05) between the NRT1.1 cRNA-injected oocytes and NC4NcRNA-injected oocytes.

As shown in FIG. 10, both NRT1.1 cRNA-injected oocytes and NC4NcRNA-injected oocytes were found to take up more nitrate thanwater-injected oocytes. Moreover, NC4N cRNA-injected oocytes were foundto take up more nitrates than NRT1.1 cRNA-injected oocytes. This resultsuggests that the chimera fused protein of the present invention hasbetter transport activity than any known NRT transports. One can expectto enhance growth or resistance to nitrogen starvation of a plant bytransforming such chimera gene with said plant.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method for changing nitrogen utilization efficiency in a plantcomprises regulating the expression of Arabidopsis NRT1.7 or anorthologue thereof so that the nitrate remobilization from older leavesto young leaves in the plant is regulated, thereby the nitrogenutilization efficiency is changed.
 2. The method of claim 1, whichcomprises transforming a plant with an expression construct with a highlevel of expression of Arabidopsis NRT1.7 or an orthologue thereof toenhance the nitrogen utilization efficiency.
 3. The method of claim 1,which comprises transforming a plant with an expression construct with alow level of expression of Arabidopsis NRT1.7 or an orthologue thereofto decrease the nitrogen utilization efficiency.
 4. The method of claim2, wherein the expression of NRT1.7 or an orthologue thereof is enhancedby an enhancer.
 5. The method of claim 2, wherein NRT1.7 isoverexpressed in a plant by transforming an expression constructcomprising a nucleic acid sequence of NRT1.7 and an enhance into theplant to produce a transgenic plant.
 6. The method of claim 4, whereinthe enhancer is 35S enhancer.
 7. The method of claim 5, wherein the 35Senhancer has a nucleotide sequence of SEQ ID NO:
 4. 8. The method ofclaim 3, wherein the quantity or activity of NRT1.7 is decreased or theexpression of NRT1.7 is inhibited to causes a defect in remobilizingnitrogen from older leaves to younger leaves so as to retard growth inthe plant.
 9. The method of claim 3, wherein the expression of NRT1.7mRNA is inhibited to decrease quantity or availability of functionalNRT1.7.
 10. A transgenic plant, which is transformed with an expressionconstruct causing overexpression of NRT1.7 or enhancement of NRT1.7function within the transgenic plant, whereby the transgenic plantenhances the nitrogen remobilization from older leaves to young leavesin the plant, and the nitrogen use efficiency is improved.
 11. Thetransgenic plant of claim 10, wherein the growth of the transgenic plantis enhanced.
 12. A transgenic plant, which is transformed with anconstruct that has a defect in the gene of NRT1.7, wherein the defectresults in inhibiting the expression of NRT 1.7 or NRT1.7 mRNA todecrease quantity or availability of functional NRT1.7, whereby thenitrogen remobilization from older leaves to younger leaves in thetransgenic plant is defective.
 13. The transgenic plant of claim 12,wherein the growth of the transgenic plant is retarded.
 14. A chimeranitrate transporter protein having the amino acid sequence of SEQ ID NO:11.
 15. An isolated DNA molecule encoding the chimera nitratetransporter protein having the amino acid sequence of SEQ ID NO: 11 ofclaim
 14. 16. The DNA molecule of claim 15 comprising the nucleotidesequence of SEQ ID NO:10.
 17. An expression vector comprising the DNAmolecule having the nucleotide sequence coding for NRT1.7 operativelyliked to a promoter.
 18. The expression vector of claim 17, wherein thepromoter is a NRT1.7 promoter.
 19. The expression vector of claim 17,which further comprises an enhancer.
 20. The expression vector of claim19, wherein the enhancer is 35S enhancer.
 21. A transgenic plant, whichis transformed with an expression construct comprising a DNA moleculeencoding the chimera nitrate transporter protein having the amino acidsequence of SEQ ID NO:11, whereby the nitrogen utilization efficiency inthe transgenic plant is improved.
 22. The transgenic plant of claim 21,wherein the DNA molecule is driven by a NRT 1.7 promoter.